Science Questions

How do we use light to work out that the universe is expanding?

Tue, 6th Sep 2016

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maurice westerhuis asked:

i had a question concerning a subject you were talking about-size of the universe


I did not have education but ..don't think that's the reason that I don't get it--'dark' (mysterious) energy (do we know by any measure, a ratio how much this speed has increased?.. meanwhile taking with this speed objects further and further away from us? )pushes the universe with increasing speed wider and wider-expanding practice,meaning that when an object has emitted light , let's say 10 billion light yrs ago and we receive the light now (and know thus by the composition of that light it was 10 billion yrs ago and the distant is as well)....all those light years when its light was on its way to us...that object has moved away from us with increasing speed. that means in my view that we cannot state and determine the size and age of the Universe---we have only by the light that has received--do we know the speed of expansion at the edge of the Universe we can perceive?

Can somebody brainwash me, so that I can think clean and clearly? Greetz from fremantle


We put this question to PhD student Adam Townsend...NGC 6744

Adam -   So, most of us know what's called the Doppler Effect. So this is when a fire engine or a police car or an ambulance, and I'm familiar with all of these because I live in London. It comes straight past your window and I and you hear it go. And like, “Neno, neno, nenooooo, ….” As it comes pass you, that’s a great sound effect though – as it comes past you, the pitch drops, the frequency drops. The same thing happens with light. Therefore, as the stars move away from us – I think Hubble actually spotted this. Hubble was looking at sort of nearby stars and spotting what they were made of by looking at their colours. And then he looked at stars that were slightly further away and found that they were sort of similar but everything was just a little bit more red. And then he looked at ones that were really, really far away. And saw that they were even more red. Therefore, he was able to sort of see that okay, everything is being shifted towards the red spectrum. This is called red shift.

Kat - Red light has longer wavelength than blue light.

Adam - Right. So then basically, by looking at the change in the colour of light where it’s like the galaxies are going past us going….

Adam - Yeah and in the same way that say, as the fire engine, goes past the frequency has decreased, they found that it went more red so the frequency has decreased.


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I'm confused here. The question in the subject line is different than the question that you're asking in the post. Which question do you want an answer to? Both? If so then we know that the universe is expanding due to cosmological redshift of galaxies. The redshift means that the galaxies are moving away from us. It's also true that the further away the galaxy is the faster its moving away. These observations are true not just for observers in our galaxy but for observers in all galaxies. There are galaxies with peculiar velocities meaning that a few of those which are nearby are moving towards us due to gravitational attraction. But that's not true for the overwhelming majority of them. This is actually what it means when its said that the universe is expanding. Think of a crowd of people who are standing close to each other. Then all of a sudden they are all moving away from each other just as if they were all standing on a rubber sheet which was being stretched uniformly. Then you'd deduce that the crowd was expanding.

As for the age of the universe, we don't know its exact age. Best estimates give a range between 10 to 20 billion years old. PmbPhy, Wed, 20th Jul 2016

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